American football is played by two teams of 11 players each. Each play involves some high-intensity, short-duration activity, and there is a short rest between plays. Professional football games have four 15-minute quarters with a halſt ime break, but the clock is frequently stopped so the game takes about 4 hours to complete. Body composition varies by position, with receivers being lean and fast, whereas line- men depend on their strength and body mass to block. College football players usually have a smaller body mass than professional players and must increase size if they move into the professional ranks. Learn more about the sport at the National Football League website (www.nfl .com).